Method of treating the area surrounding acid gas storage wells

ABSTRACT

Method of treating the area surrounding an acid gas storage well ( 1 ) using a reactive solution, wherein the following stages are carried out:
         a) injecting from the well a wash fluid for washing the rock of said surrounding area so that it no longer contains products reactive with said solution,   b) injecting into the rock thus washed a predetermined volume of reactive solution suited to react with acid gases.

FIELD OF THE INVENTION

The present invention relates to the sphere of geological storage ofacid gases, including CO₂. In particular, the invention relates to theclosing of wells giving access to geological formations wherein storageis achieved through injection. One of the goals is to prevent leaking ofacid gas, CO₂ for example, through said wells or the surrounding area.

BACKGROUND OF THE INVENTION

Well abandonment procedures wherein plugs of various qualities:mechanical plugs made of an expandable material, cement plugs, resinplugs, are fed into well casings are known. However, the durability ofthese materials and of the casing pipe subjected to corrosion is notsufficient in the case of acid gas storage, notably CO₂.

Forced injection (squeeze) of specific plugging products through wellperforations in order to seal the porous and permeable formation is alsoknown. However, setting of the products injected is difficult tocontrol, which makes the plugging efficiency unpredictable.

SUMMARY OF THE INVENTION

The present invention thus relates to a method of treating the areasurrounding an acid gas storage well using a reactive solution, whereinthe following stages are carried out:

a) injecting from said well a wash fluid for washing the rock of saidsurrounding area so that it no longer contains products reactive withsaid solution,b) injecting into the rock thus washed a predetermined volume ofreactive solution suited to react with acid gases, said solutioncomprising basic oxides so as to precipitate minerals into the rock, incontact with the acid gases.

According to the method, acid gas can be stored prior to stages a) andb).

The reactive solution can be suited to precipitate carbonates, hydrogencarbonates or sulfides.

The basic oxides can be selected from the following group: alkaline oralkaline-earth oxides, their hydrated forms, Zn, Ti, Mn, Fe, Zr oxidesor their admixtures.

The basic oxides can come from the following minerals: basic andultrabasic rocks, such as basalts, serpentinites, peridotites,magnesite, possibly calcined, dolomite, albite, cements, hydrated ornot, ultrafine cements, blast furnace slag, geopolymers, alkalinesilicates, wollastonite, pouzzolanic materials, plaster, clinker, talc,kaolin, other clays, possibly calcined, silica fumes, fly ashes,zeolites.

The well can be plugged after injecting the reactive solution.

Stages a) and b) can be carried out after drilling into the geologicaloverburden zone overlying said gas storage site so as to reduce or toplug the possible acid gas leaks in said zone.

The reactive solution can comprise oxides of predetermined grain sizedepending on the nature of the porous medium injected.

The reactive solution can comprise rheological property control agentssuch as hydrosoluble polymers, associative polymers, clays.

The basic oxides can be colloidal particles.

The wash solution can be aqueous.

The density and the flow properties of the wash solution can bedetermined for optimized displacement of the acid gases.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the present invention will be clearfrom reading the description hereafter of embodiments given by way ofnon limitative examples, with reference to the accompanying figureswherein:

FIG. 1 diagrammatically shows an embodiment of the invention, and

FIG. 2 also diagrammatically shows another embodiment of the invention.

DETAILED DESCRIPTION

The principle of the invention is based on the setting of a materialreactive upon contact with acid gas in the area surrounding a wellgiving access between the surface of the ground and the geological acidgas storage reservoir. The result of the reaction is to greatly limitthe permeability of the reservoir rock and even to plug it in said areasurrounding the well.

FIG. 1 shows a wellbore 1 drilled through cap rock 2 that overliesreservoir rock 3. Wellbore 1 is cased by a tube 4 cemented in borehole 1by a cementing material 5. Access to the reservoir is obtained through adrilled drain 6. An injection tube 7 ended by a strainer 8 is loweredinto the well and the annulus between said injection tube and casing 4is sealed by seal means of packer 9 type or equivalents, well known inthe trade.

The diagram of FIG. 1 shows a non-limitative well equipment example,other variants being applicable to the present invention, in particularcompletions or well equipments for horizontal wells.

Acid gas injection is carried out by means of tube 7. Once filling ofthe reservoir is completed, a washing operation is carried out so as todrive the CO₂ or the H₂S out of zone 10 to be treated. Water ispreferably used, but other fluids can also be used for washing insofaras they fulfil equivalent functions. For example, viscosifying additivescan be added to improve washing.

The volume of wash fluid must be sufficient to drive the acid gas by aradial distance of at least some meters away from the area surroundingthe well. This flushing, preferably with water, ensures thereafter goodinjectivity of the formulation comprising the reactive material in thearea surrounding the well. In the absence of flushing, the method couldbe ineffective because there is a risk of fast formation of superficialmineral compounds (carbonates and/or sulfides), which may locally causepore clogging, thus limiting encroachment of the well surroundings bythe reactive formulation.

After washing, a formulation comprising the products reactive to acidgases is then injected into the area surrounding the well and within aradius of some meters. Arrows 11 schematize said injection.

In case of circulation of CO₂ (either in supercritical form or inaqueous solution) or of acid gas in the area surrounding the well, thegoal of the reactive material is to mineralize this acid gas by inducinga great permeability reduction of the surrounding area, which notablyallows to protect the well against an acid gas attack.

This reactive material for the present invention is selected from amongthe basic oxides of generic formula MxOy, with M an element selectedfrom among alkaline, alkaline-earth or other elements, characterized inthat, in the presence of water, they dissociate at least partly and formhydroxyl ions, and in that they react in the presence of acid gases suchas CO₂ or H₂S, by forming respectively weakly water-soluble carbonatesor sulfides.

The alkaline or alkaline-earth oxides are preferably selected notablyfrom among sodium, potassium, calcium, barium and magnesium oxides, ortheir hydrated form (LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2,Sr(OH)2,Ba(OH)2, Mg(OH)2), as well as Zn, Ti, Mn, Fe, Zr oxides.

These oxides can come in form of mineral particles, the minerals beingselected from among basic and ultrabasic rocks (such as basalts,serpentinites, peridotites, known to be rich in magnesium), magnesite,possibly calcined, dolomite, albite, cements (hydrated or not),ultrafine cements, blast furnace slag, geopolymers, alkaline silicates,wollastonite, pouzzolanic materials and other binders in general,plaster, clinker, talc, kaolin, other clays, possibly calcined, silicafumes, fly ashes, zeolites.

Materials with large proportions of magnesium and/or calcium oxides thatform very stable carbonates in the presence of CO₂ are preferablyselected.

Examples of carbonation reactions of various calcium and magnesiumoxides are given hereafter:

Calcium  oxide:CaO + CO₂ → CaCO₃Magnesium  oxide:MgO + CO₂ → MgCO₃Calcium  hydroxide  (lime):Ca(OH)₂ + CO₂ → CaCO₃ + H₂OMagnesium  hydroxide:Mg(OH)₂ + CO₂ → MgCO₃ + H₂O$ {{{Talc}\text{:}\frac{1}{3}{Mg}_{3}{Si}_{4}{O_{10}({OH})}_{2}} + {CO}_{2}}arrow{{MgCO}_{3} + {\frac{4}{3}{SiO}_{2}} + {\frac{1}{3}H_{2}O}} $$ {{{Forsterite}\text{:}\frac{1}{2}{Mg}_{2}{SiO}_{4}} + {CO}_{2}}arrow{{MgCO}_{3} + {\frac{1}{2}{SiO}_{2}}} $Wollastonite:CaSiO₃ + CO₂ → CaCO₃ + (SiO)₂

When the acid gas tends to return to the well, these bases react withthe acid gas coming from the storage reservoir and mineralize it in thepores of the porous medium surrounding the well in form of carbonates orhydrogen carbonates in the case of CO₂, and in form of sulfides in thecase of H₂S, thus reducing the porosity and the permeability in thesurrounding area, and therefore decreasing the potential leak rate ofacid gas to the surface through the well. The precipitated minerals alsoform a protective layer for the well equipments against the acid gasesstored.

These bases are brought into aqueous solution, or even oversaturatedsolution, i.e. in suspension. In the case of suspensions, the grain sizehas to be adjusted and controlled in relation to the porosity of themedium, because the size of the particles must be small enough to enterthe porous medium. This is provided through rigorous selection of theoxides (or oxide mixtures) that make up the formulation, and gooddispersion thereof. The D50 of the particles selected preferably rangesbetween 0.2 and 200 μm, the D90 must be less than ⅓ of the mean diameterof the pores of the formation. A formulation comprising a mixture ofgrain sizes can be preferably used.

The specific surface area of the oxides selected is maximized so as tohave a maximum amount of reactivity.

The rheology of the suspension thus formed is also adjusted in order toensure high-performance injection while avoiding fracturation. Accordingto the characteristics of the rock formation into which injection isperformed, the viscosity of the suspension is adjusted so that theinjection pressures range between the pore pressure and the fracturepressure of the medium. The viscosity is also adjusted so as to maintainsaid particles in suspension throughout injection. Viscosities between 1mPas and 1 Pa·s are sought.

Rheology modifying additives are therefore advantageously added, notablyhydrosoluble polymers stable in basic media, of sufficiently high molarmass to guarantee a viscosity effect with a low volume fraction ofpolymeric additive in the auto-blocking formulation.

Vinyl polymers carrying carboxylate, sulfonate or phosphate groups canbe used, for example polyacrylates of molar mass above 10⁶ g/mol, orsulfonated polymers such as polystyrene sulfonate, polynaphthalenesulfonate.

It is also possible to add colloidal particles, for example swellingclays such as bentonites, smectites, whose sheets disperse in basicaqueous phase and whose function is to confer on the fluid a yield pointthat is useful for the efficient suspension of mineral particles (suchas basic oxides under oversaturation).

Furthermore, the density of the formulation is adjusted according to thecharacteristics of the medium (rock fracture pressure), with respect tothe hydrostatic pressure. The oxides are therefore selected, or theirmixtures are adjusted, according to their respective density and volumefraction. The aqueous phase can contain dissolved salts to increase thedensity of the water, as it is commonplace in conventional drillingfluids (CaCl2, NaCl, . . . ).

The formulation can comprise weighting agents such as salts (bariumsulfate or others), weakly soluble and maintained in the dispersedstate. Their grain size must be adjusted and controlled, with respect tothe porosity of the medium, and the size of the particles must be smallenough to enter the porous medium.

The state of dispersion of the formulation can be controlled by addingdispersing agents such as surfactants, preferably anionic, such as alkylsulfonates, alkyl phosphates, alkyl carboxylates, or non ionic, such asalkylated polyoxyethylenes.

The invention first applies to the closing of an injector well that isno longer going to be used, which can be likened to the abandonment ofan oil well. The invention can however also be used during theconstruction of a well specifically drilled for geological acid gasstorage, whether in reservoir rocks, aquifers or coal veins.

After acid gas injection, water, or more generally the wash fluid, isinjected through acid gas injection tube 7 so as to drive the storedacid gas far away from the area surrounding the well. In case of acidgas storage in a saline aquifer, the water used as the flush fluid cancome from this aquifer, and it may have been produced during injectionof the acid gases. Once this operation achieved, the reactiveformulation is injected through injection tube 7 and squeezed in therock formation. The volume injected (squeezed) is suited to invade somemeters in the area surrounding the well.

The operation is ended by injecting a cement plug, or any other pluggingformulation, so as to maintain the reactive material in place. When thisfirst operation is completed, it is possible to carry out the sameoperation in other zones, in particular the top of the reservoir, afterperforating the casing and the primary cementing so as to allowinjection.

The invention also applies when drilling a new well in order to injectacid gases to be stored therein. In this case, once the cap rock of thereservoir reached and traversed by the wellbore, the aforementionedmethod is applied. FIG. 2 shows a wellbore 21 that has reached cap rock20 overlying storage reservoir 22. A water flush operation, theninjection, just below the level of the overburden (top of thereservoir), of a reactive formulation is carried out. A string of tubes23 and an annulus insulating packer 24 are therefore used. Zone 25 isthus invaded by a reactive material that will react in case of acid gasleaking in said zone. Drilling is continued with the drilling fluid tothe desired depth, then casing and cementing is carried out, prior toperforming the perforations required for subsequent injection of theacid gas(es). This preventive method, carried out upstream from the CO₂injection, allows to limit leak risks at the level of the overburden,where the CO₂ plume might accumulate during and after injection, thusgreatly reducing the permeability of the reservoir rock just below theclay layer. During well closing for abandonment, the procedure describedabove is applied.

1) A method of treating the area surrounding an acid gas storage wellusing a reactive solution, wherein the following stages are carried out:a) injecting from said well a wash fluid for washing the rock of saidsurrounding area so that it no longer contains products reactive withsaid solution, b) injecting into the rock thus washed a predeterminedvolume of reactive solution suited to react with acid gases, saidsolution comprising basic oxides so as to precipitate minerals into therock, in contact with the acid gases. 2) A method as claimed in claim 1,wherein acid gas has been stored prior to stages a) and b). 3) A methodas claimed in claim 1, wherein the reactive solution is suited toprecipitate carbonates, hydrogen carbonates or sulfides. 4) A method asclaimed in claim 1, wherein the basic oxides are selected from thefollowing group: alkaline or alkaline-earth oxides, their hydratedforms, Zn, Ti, Mn, Fe, Zr oxides or their admixtures. 5) A method asclaimed in claim 1, wherein the basic oxides come from the followingminerals: basic and ultrabasic rocks, such as basalts, serpentinites,peridotites, magnesite, possibly calcined, dolomite, albite, cements,hydrated or not, ultrafine cements, blast furnace slag, geopolymers,alkaline silicates, wollastonite, pouzzolanic materials, plaster,clinker, talc, kaolin, other clays, possibly calcined, silica fumes, flyashes, zeolites. 6) A method as claimed in claim 1, wherein the well isplugged after injecting the reactive solution. 7) A method as claimed inclaim 1, wherein stages a) and b) are carried out after drilling intothe geological overburden zone overlying said gas storage site so as toreduce or to plug the possible acid gas leaks in said zone. 8) A methodas claimed in claim 1, wherein the reactive solution comprises oxides ofpredetermined grain size depending on the nature of the porous mediuminjected. 9) A method as claimed in claim 1, wherein the reactivesolution comprises rheological property control agents such ashydrosoluble polymers, associative polymers, clays. 10) A method asclaimed in claim 1, wherein the basic oxides are colloidal particles.11) A method as claimed in claim 1, wherein the wash solution isaqueous. 12) A method as claimed in claim 1, wherein the density and theflow properties of the wash solution are determined for optimizeddisplacement of the acid gases.